How-To

Hydroponic Troubleshooting Guide

JeffGrow Researcher

Updated 10 March 2026

Home grower and obsessive researcher. Years in commercial product sourcing means I evaluate growing equipment the way a buyer does — specs, build quality, and real-world performance, not marketing claims.

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Plants communicate through their leaves. Learning to read these signals transforms troubleshooting from guesswork into diagnosis. This guide covers the problems you'll actually encounter.

## Quick Diagnosis Chart

Symptom	Most Likely Cause	First Check
Yellow older leaves	Nitrogen deficiency or pH	Check pH
Yellow younger leaves	Iron deficiency	Check pH (usually too high)
Brown leaf tips	Nutrient burn	Check EC/TDS
Brown/slimy roots	Root rot	Check water temp, oxygen
Wilting with wet roots	Root rot	Check roots directly
Slow growth	Low light or low nutrients	Check light hours, EC
Leggy/stretchy plants	Insufficient light	Add light or move closer
Algae in reservoir	Light reaching solution	Block all light

The honest truth: 90% of problems trace back to pH. Always check pH first. It's the cause more often than you'd expect.

## Yellow Leaves

The most common question. Yellowing means something, but what depends on which leaves and how they yellow.

Older leaves yellowing (bottom of plant first): Usually nitrogen deficiency. Plants move nitrogen from old leaves to new growth when supply is short.

Fix: Check pH first (nitrogen lockout above pH 7). If pH is fine, increase nitrogen or overall nutrient strength.

Younger leaves yellowing with green veins: Iron deficiency (interveinal chlorosis). Almost always caused by high pH - iron becomes unavailable above pH 6.5.

Fix: Lower pH to 5.5-6.0. Iron deficiency corrects within days once pH is right.

All leaves pale yellow-green: General nutrient deficiency. Either solution is too weak or pH is locking out multiple nutrients.

Fix: Check EC/TDS. If adequate, check pH. Fresh solution often resolves this.

Yellow spots or mottling: Various causes - could be potassium deficiency, calcium deficiency, or pest damage. Look for other clues.

## Root Problems

Healthy roots are white or cream-coloured, firm, and smell neutral. Any deviation is worth investigating.

Brown, slimy roots: Root rot. Caused by pathogens that thrive in warm, low-oxygen conditions.

Causes: Water too warm (above 22C), insufficient oxygen (DWC without adequate bubbling), stagnant water, contaminated equipment.

Fix: Remove affected plants. Clean system thoroughly. Start fresh with cooler water. Add more aeration. Consider beneficial bacteria products like Hydroguard.

Brown but firm roots: Nutrient staining. Some fertilisers dye roots brown. This is cosmetic, not harmful.

How to tell: Stained roots stay firm and functional. Rotted roots are slimy and smell bad.

Minimal root growth: Could be too-strong nutrients (roots burn back), too-weak nutrients (nothing to grow toward), or environmental stress.

Fix: Check EC (should be appropriate for growth stage). Check water temperature (18-22C ideal). Ensure light isn't reaching roots.

## Algae

Green growth on surfaces, in solution, or on growing medium. More nuisance than crisis but worth addressing.

Cause: Light reaching nutrient solution. Algae photosynthesises like plants - give it light and nutrients and it grows.

Problems: Competes with plants for nutrients. Can clog drippers and lines. Creates smell. Looks bad.

Prevention: - Use opaque containers - Cover reservoirs completely - Block light from reaching any solution - Black tape over any light leaks

Treatment: Remove affected solution. Clean surfaces. Block light sources. Start fresh.

## pH Problems

pH rising constantly: Plants consuming nutrients faster than they consume water. The solution becomes more concentrated and alkaline as the buffering capacity depletes.

Fix: Change solution more frequently. Consider using larger reservoir for more stable chemistry.

pH dropping constantly: Less common. May indicate bacterial activity in reservoir or decomposing organic matter.

Fix: Clean system thoroughly. Check for dead roots or decaying plant material. Change solution.

pH won't adjust: Hard water with high mineral content resists pH change. Your area's water is heavily buffered.

Fix: Use more pH adjuster (carefully). Consider rainwater or filtered water for some of your mix.

## Wilting

Wilting with dry reservoir: Simple - plants need water. Refill.

Wilting with plenty of water: Root damage. Plants can't transport water despite it being available. Check roots for rot.

Wilting in hot conditions: Heat stress or transpiration exceeding uptake. Increase airflow, reduce temperature if possible, ensure roots have access to water.

Wilting that recovers overnight: Often normal on hot days. Plants wilt during high transpiration and recover when it's cooler. Mild wilting in afternoon heat isn't necessarily a problem if plants look fine by morning.

## Slow Growth

Check light first. Insufficient light is the most common cause of slow growth indoors. Herbs need 12-16 hours of adequate light. Move closer to light source or add more lighting.

Check temperature. Growth slows below 15C and above 30C. Most vegetables prefer 18-25C.

**Check nutrients.** Low EC limits growth potential. If light and temperature are fine, increase nutrients slightly.

Check roots. Root problems restrict nutrient uptake regardless of what's in the solution.

## Pests

Fungus gnats: Small flies whose larvae eat roots. They love wet growing media.

Fix: Let media surface dry between waterings (if using media). Use yellow sticky traps. Sand layer on surface deters egg-laying.

Aphids: Appear seemingly from nowhere. Check new plants carefully before adding to your system.

Fix: Blast off with water spray. Use insecticidal soap. Introduce ladybirds if growing in a greenhouse.

Spider mites: Tiny, cause stippled leaves. Thrive in hot, dry conditions.

Fix: Increase humidity. Spray with water. Use neem oil or insecticidal soap.

## What to Avoid

Changing multiple things at once: Change one variable, wait 3-5 days, observe. Changing everything simultaneously means you won't know what worked.

Panicking: Plants are resilient. Most problems are fixable if caught early. A few yellow leaves aren't a crisis.

Overwatering media-based systems: Just because it's hydroponics doesn't mean roots should be constantly wet in flood-and-drain or drip systems.

**Adding more nutrients to fix problems:** Usually makes things worse. Diluting is often better than adding.

## The Diagnostic Process

1. Check pH (it's almost always pH) 2. Check EC/TDS 3. Check water temperature 4. Check roots directly 5. Check light levels 6. Check for pests

This order catches 95% of problems. Work through systematically rather than guessing.

Bluelab

pH Test Kit with Adjustment Solutions

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For step 1, our pH guide has everything you need. For step 2, the [nutrients guide](/guides/hydroponic-nutrients-guide) explains EC targets and deficiency symptoms in detail.

Take our quiz if you want recommendations for your specific setup.

The diagnostic process in hydroponics is worth genuinely learning rather than just following as a checklist. When you understand why pH above 7 locks out nitrogen, or why warm water holds less dissolved oxygen and creates root rot conditions, the checklist becomes intuition. You stop guessing and start reading your plants. That shift — from reactive troubleshooting to proactive management — is what separates growers who struggle with the same problems repeatedly from those who catch issues before they become problems.

## Common Mistakes to Avoid

Diagnosing by symptom alone without testing: Yellow leaves have multiple causes. Brown tips have multiple causes. Always test before treating — check pH and EC before adding anything to the reservoir. The right diagnosis is worth five minutes of testing; the wrong treatment can set you back weeks.

Treating symptoms rather than causes: Root rot comes from warm, oxygen-depleted water. Fixing the roots without addressing the water temperature means it comes back. The symptom is the root rot; the cause is the environment that allowed pathogens to thrive.

Overcorrecting: If pH is 7.2, don't dump in pH Down to get it to 5.8 in one adjustment. Add in small increments, mix thoroughly, retest. Large swings stress plants as much as sustained out-of-range pH. Aim for gradual correction toward the target range.

Our [beginner's guide](/guides/hydroponic-beginners-guide) covers preventive practices that reduce how often you'll need this guide.

## Root Rot: The Most Serious Problem

Root rot kills plants faster than any other issue. When it takes hold, healthy white roots turn brown and slimy with a distinctly unpleasant smell. The plant wilts even though it's sitting in water — because the rotted roots can't absorb anything.

Causes: - Water temperature above 23°C (74°F) — warm water holds less dissolved oxygen and bacteria thrive - Insufficient aeration — stagnant water allows pathogens to multiply - Light entering the reservoir — algae growth creates ideal conditions for root pathogens - Dirty system — old organic matter harbours disease

Treatment: 1. Empty and clean the reservoir with a dilute hydrogen peroxide solution 2. Trim away heavily affected roots with clean scissors 3. Reduce water temperature — add a frozen water bottle if needed 4. Add beneficial bacteria (Hydroguard or similar) to outcompete pathogens 5. Ensure no light reaches the root zone 6. Increase aeration

Prevention is easier than treatment. Keep water below 21°C, ensure complete light exclusion, run adequate aeration, and add beneficial bacteria as a precaution.

## Pest Problems

Hydroponics is generally less pest-prone than soil growing, but it's not immune. The most common issues:

Fungus Gnats Small flies whose larvae live in growing media and attack roots. More common in systems using coco or rockwool than in pure water culture.

Prevention: Use yellow sticky traps (catch adults). Allow growing media to dry slightly between waterings if applicable. Beneficial nematodes treat larvae in substrate.

Aphids Cluster on new growth. Distort leaves and leave sticky honeydew deposits. In an enclosed grow space, populations explode quickly.

Treatment: Spray with dilute neem oil solution or insecticidal soap. Remove severely affected leaves. Ladybirds are natural predators if you're growing in an open greenhouse.

Spider Mites Tiny dots on leaf undersides. Fine webbing in severe infestations. Thrive in hot, dry conditions.

Prevention: Maintain humidity above 50% and keep temperatures moderate. Treat with neem oil or specialist miticides.

The best pest strategy: Inspect plants regularly. Catch problems early. An infestation caught at five insects is manageable; at five hundred, it's a crisis.

## Algae in the Reservoir

Green or brown growth in your reservoir or tubing means light is reaching nutrient solution. Algae competes with plants for nutrients and creates conditions that encourage root rot.

Fix: Block every light source from the reservoir. Check pump tubing, net pot holes, and reservoir lids. Use opaque containers from the start — retrofitting light exclusion is annoying.

Black containers prevent this entirely. If you're using a clear container, paint it or wrap it in opaque tape.

## Monitoring Schedule

Consistent monitoring prevents small issues from becoming big ones.

Every session (when you visit plants): - Visual check: leaf colour, plant stance, any wilting - Check reservoir level — top up with pH-adjusted water as needed

Every 2-3 days: - Test pH — drift is normal, especially in active systems - Check EC/TDS if you're not doing full reservoir changes

Weekly: - Full reservoir change for recirculating systems - Check roots if accessible — any browning? - Clean and inspect pump/airstone

Monthly: - Deep clean reservoir and tubing - Check growing media condition

## Frequently Asked Questions

Why are my plant leaves curling?

Curling inward (cupping) usually means heat or moisture stress. Check temperature and humidity. Curling under at the edges can indicate calcium deficiency or overfeeding. Curling up at the edges often points to high nitrogen.

My plants are growing slowly. What's wrong?

Rule out the basics first: pH in range, EC adequate, sufficient light hours, temperature between 18-26°C. Slow growth in winter often just means lower light intensity — not a system problem.

Can I reuse nutrient solution?

For recirculating systems, yes — top up regularly and do full changes every 1-2 weeks. For Kratky, it's easier to top up with fresh solution when levels drop. Very old solution can develop microbial issues or nutrient imbalances.

How do I know when to change the reservoir?

With recirculating systems, change the full reservoir every 1-2 weeks for active growing. In Kratky systems, top up as water depletes and do a full refresh every 3-4 weeks or between crops.

Something's wrong but I can't identify it. What should I do?

Start with the checklist: pH → EC → water temperature → root inspection → light levels → airflow. Work through systematically. The answer is almost always in those six checks. If you've ruled everything out, take a photo and post to a hydroponics forum or subreddit — fresh eyes often spot what you've been staring past.

Start with your conditions, not your symptoms, and you'll solve problems in minutes rather than days.